The use of implants to restore the function of traumatized or degenerated connective tissues and thus to improve the quality of life of a patient has become widespread. The long-term success of dental implants largely depends on rapid healing with safe integration into the jaw bone and soft tissue. The integration of the dental implant intra-osseus component and the transmucosal component (abutment) with the hard (bone) and the soft (gingiva) tissue respectively, is essential in order to minimize dental implant failure.
Osseointegration is defined as the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant. Current research of biomaterials is focused on inducing said physiological event by selecting a proper chemical or biological molecule able to provoke it. On the other hand, abutment surface properties may promote gingival fibroblasts attachment and synthesis of collagen-rich connective tissue, providing a tighter seal around the abutment, avoiding bacterial penetration and downgrowth of gingival epithelial cells.
Titanium (Ti) and its alloys are the materials most frequently used as bone implants as they combine good mechanical properties, such as high strength, high toughness, and low density, with good biocompatibility, caused by biological inertness due to a chemically stable surface oxide layer and an elastic modulus closer to that of bone than ceramics or steel. Ti implants are applied in various sites; in the jaw as dental implants and abutments for the bone and soft tissue respectively, as plates, screws, pins and wires to facilitate bone healing, and as prostheses for knee, hip, and other joints. Ti is commonly used in dental and orthopaedic applications but also in vascular stents.
The surface of Ti is only bioinert, thus current research on modification of implant surfaces focuses on making virtual bioinert materials become bioactive. The assortment of surface modifications ranges from non-biological coatings, such as carbide, fluorine, calcium, hydroxyl apatite or calcium phosphate, to biological coatings that bind different biomolecules to the implant surface. Such binding has often been carried out using for example chemical reactants such as formalin or glutaraldehyde, but the reactive nature of these agents often leads to the biomolecules becoming biologically inactive and/or with enhanced immunoreactivity, which is of course undesirable.
Biomolecules can be immobilized through a variety of procedures such as adsorption, covalent coupling, electrochemical surface modifications and self organized organic layers on the implant surface. It is known that oxide layers on titanium based materials show isoelectric points around 4.1 indicating that these oxide layers carry a negative charge under in vivo conditions. Thus, macromolecules positively charged under these conditions should adsorb due to electrostatic interactions.
Adsorptive binding methods combine the advantage of being simple and applicable to a nearly unlimited extent. Drawbacks are the rather low stability of biomolecule fixation, a non-defined release behaviour of biomolecules and possible conformational changes of the directly adsorbed molecules.
The advantage of covalent binding is the stable fixation of the biomolecule, combined with the chance to preserve biological activity to some degree if the molecule is combined with linkers/spacers of sufficient length. However, if more than one biomolecule is considered for immobilization, it is difficult to combine different molecules in a defined way on the surface. Thus, there is a need in the prior art to conceive covalent surface attachment strategies that are successful, with stable coatings that keep the biochemical activity of the biomolecule, in comparison with physical adsorption that is not successful for long-term implantation mainly due to the desorption of biomolecules.
As selected antioxidants, both flavonoids and methoxytryptophols have never been reported to be covalently attached to the metal surface of an implant, directly or through a linker. Moreover, methoxytryptophols have never been reported in the context of bone regeneration or implants and surface coatings for improved osseointegration and soft tissue attachment.
Flavonoids are natural phenolic compounds present in fruit and vegetables with antioxidant and many biological functions, including osteogenic, anti-osteoclastogenic and anti-adipogenic effects. Besides the osteogenic capacity of these biomolecules, another property that might be beneficial is their antimicrobial effects. Flavonoids have previously been described in US 2010/0068238A1 in the context of biocompatible medical implants, such as stents, that comprise a composition for controlled delivery of flavonoids or a derivative thereof for prevention of restenosis. However, there is no mention of the use of flavonoids for improved osseointegration or soft tissue attachment. The coating process is based on the use of bioresorbable polymers for controlled release and does not mention the use of covalent attachment for permanently binding flavonoids to the implant surface.
Furthermore, US 2011/0112654A1 discloses a bone implant coated or impregnated with flavones, which presents the ability of promoting osseointegration, as well as reducing inflammation. However, the application is based on the use of lactoferrin in dip-coated titanium implants, which can be combined with other substances including flavones, for the reduction of inflammation, increased osseointegration and reduction of adherent bacteria. Thus, it does not mention the use of covalent attachment for permanently binding flavonoids to the implant surface.
In US 2008/0241211A1 is described a medical appliance for bone regeneration that includes an osteinductive enhancer such as a flavonoid. However, the application describes the use of flavonoids in bone graft applications with demineralized bone matrix and collagen sponges, and does not describe the use of flavonoids covalently attached to implant surfaces. A similar application is described by Wong et al. in Biomaterials 27 (2006) 1824-1831 with the use of the flavonoid naringin in collagen bone grafts. Thus, a flavonoid solution is mixed with the collagen matrix and does not include the use of implant surfaces or a method for covalently binding flavonoids to metal implant surfaces.
Methoxytryptophols like 5-methoxytryptophol and 6-methoxytryptophol among others, are indole compounds that are neither a melatonin metabolite nor its precursor, and possess antioxidant properties. Their effects on bone or coated on an implant have never been reported before.
Despite the availability of biocompatible implants in the art today, there is still a need to identify alternative biocompatible implants which further may facilitate osseointegration of an implant when introduced into a mammalian body. Furthermore, there is a limitation of most of the available techniques with regards to the physical absorption (labile union) of these compounds onto the surface of the implant.